In current hard disk drives, data is written in concentric circular tracks by a magnetic recording head which is positioned above a spinning disk. The magnetic recording head consists of a write head element which is used during writing, and a read head element which is used during reading. During writing, the position of the magnetic recording head above the disk is precisely controlled by a servomechanical feedback loop so that the written data tracks do not overlap. The width of each track is determined by the width of the write head element. The center-to-center spacing of each track (also known as the track pitch) is slightly larger than the track width, and is determined by the servomechanical control system which keeps the head properly positioned above the disk. Each track is separated from its neighbor by a guard band whose width is equal to the track pitch minus the track width. During read back, the read back element in the recording head is precisely positioned above a single track. The width of the read back element is usually equal to or smaller than the width of a written track, so that the read head element detects the signal from only that single track and not from neighboring tracks.
With current hard disk drives, each track may be randomly written at any time without disturbing the data on any of the other tracks. This ability to randomly access the data on the surface of the disk is an important characteristic of hard disk drives which is not shared by all data storage systems.
In U.S. Pat. No. 6,185,063, incorporated herein by reference, a method of writing tracks in a partially overlapping fashion is described. This overlapping writing fashion can be referred to as “shingled track” writing. Specifically, with shingled track writing, data tracks are written such that each written track partially overlaps an immediately adjacent track that is contiguous to it, like shingles on a tiled roof. Thus, a first track is partially overwritten when a second track contiguous to the first is written, which in turn is partially overwritten when a third track contiguous to the second is written, and so on.
As recognized herein, unlike the conventional approach described above, the write head width advantageously can be significantly larger than the track pitch in shingled track writing; whereas the width of the read-back head element can be slightly less than the track pitch so the read back head still detects signal from only a single track and not from neighboring tracks.
The present invention recognizes that although, for reasons discussed more fully below, shingled writing can result in higher data storage density, it may not allow for random access writes to any arbitrary track, because to rewrite one track it may also be necessary to overwrite some of the information stored on subsequent adjacent tracks. The present invention recognizes that it is possible to update data on a track using a multi-step process, in which the data from neighboring tracks is copied elsewhere, the desired track rewritten, and then neighboring tracks are restored to their original state. However, it is no longer possible to update the data on any arbitrary track in a single step, and so some of the random access properties of the storage mechanism are sacrificed. As critically recognized by the present invention but not the above-referenced patent, a consequence is that a disk drive which uses shingled track writing may require a different kind of file system structure from a conventional hard disk drive.